1. Field of the Invention
The present invention generally relates to an image forming apparatus and more particularly to an image forming apparatus where waste toner can be collected without a power supply that generates a voltage for collecting waste toner from a photoconductive drum.
2. Description of the Related Art
A conventional image forming apparatus such as an electrophotographic printer and a copying machine performs an electrophotographic process. The electrophotographic process includes charging, exposing, developing, transferring, and fixing. The charging process is performed for storing charges on the photoconductive drum. The charges on the surface of the photoconductive drum are selectively dissipated in accordance with print data to form an electrostatic latent image. The electrostatic latent image is developed with toner into a toner image. The toner image is transferred onto a recording medium, and is then fused on the recording medium under heat and pressure into a permanent image.
A toner layer formed on the developing roller is attracted to the electrostatic latent image and some toner that failed to be deposited on the electrostatic latent image is returned to a toner chamber in the developing unit. The toner, returned into the toner chamber, is again used to form a toner layer on the developing roller. If a printing operation is repeatedly performed for print data having an image area in which toner occupies only very small areas on a page of the recording medium, only a small amount of toner is consumed and a large amount of unused toner is returned to the toner chamber. The unused toner is again used to form a toner layer on the developing roller. This operation is repeated many times.
Thus, the toner subjected to repetitive use is overcharged. Additionally, surface additives, which are added on the toner particles for adjusting an amount of charges and fluidity of toner, come off the toner particles. The surface additives that have come off sink in the toner particles. Thus, the toner particles clump into large agglomerates. The agglomerates of deteriorated toner particles cause non-uniform density of printed images, increased contamination of the recording medium, and missing dots, which in turn impair the print quality.
The aforementioned image forming apparatus is equipped with a mechanism that collects residual toner on the photoconductive drum resulting from normal printing operations. The mechanism discharges the collected residual toner, thereby eliminating problems resulting from deteriorated toner particles. This allows miniaturizing of a developing unit and facilitates maintenance of an image forming apparatus.
With the aforementioned conventional image forming apparatus, when the residual toner is collected, a cleaning member collects the residual toner from the photoconductive drum with the bias voltage of a transfer roller set to zero volts. Therefore, the toner deposited to the transfer roller and the surroundings within the image forming apparatus may contaminate the recording medium.
During transfer process, the transfer roller receives a bias voltage to transfer the toner image onto the recording medium. During a toner discharge process, the transfer roller receives a bias voltage of zero volts so that the residual toner remains on the photoconductive drum. The cleaning means in contact with the photoconductive drum collects the residual toner from the photoconductive drum.
However, merely setting the bias voltage of the transfer roller to zero volts is not enough to prevent the residual toner from being deposited to the transfer roller and the surroundings in the image forming apparatus. As a result, the toner image cannot be transferred thoroughly from the photoconductive drum during normal printing, and the toner deposited on the transfer roller and the surroundings can contaminate the recording medium.
The present invention was made in view of the aforementioned problems of the conventional image forming apparatus.
An object of the invention is to eliminate the need for providing a power supply that charges the transfer member during the toner discharging process.
Another object of the invention is to provide an image forming apparatus in which when residual toner is removed from an image bearing body, a charging member charges the image bearing body which in turn charges a transfer member so that toner is not deposited to a transfer member.
Another object of the invention is to provide an image forming apparatus in which residual toner is collected in a short time.
An image forming apparatus having at least one image forming section that includes a rotating image bearing body, a charging member such as a charging roller, an exposing unit, a developing unit, a transfer member such as a transfer roller, a toner removing member, and a controller. The charging member charges a surface of the image bearing body such as a photoconductive drum. The exposing unit illuminates the surface of the image bearing body charged by the charging member to form an electrostatic latent image on the surface. The developing unit holds toner therein and develops the electrostatic latent image with the toner into a toner image. The transfer member transfers the toner image from the image bearing body onto a recording medium. The toner removing member removes residual toner from the surface of the of the image bearing body after the toner image is transferred onto the recording medium. The controller performs a waste toner discharging operation. During the waste toner discharging operation, the controller controls the charging member to charge the surface of the image bearing body in such a way the charging member charges the image bearing body which in turn charges the transfer member. Then, the exposing unit illuminates the charged surface of the image bearing body. Finally, the developing unit supplies the toner to the illuminated surface, so that the toner supplied to the illuminated surface remains on the image bearing body when the toner on the illuminated surface on the image bearing body passes the transfer member as the image bearing body rotates.
The controller controls the charging member such that |Vr| greater than |Vc|, wherein Vr is a voltage applied to the charging member when the deteriorated toner is discharged, and Vc is a voltage applied to the charging member when a normal printing operation is performed.
The transfer member may be a transfer roller. During the waste toner discharging operation, at least one complete circumferential surface of the transfer roller has been charged before the toner on the image bearing body passes the transfer roller.
The transfer member may be a transfer belt. During the waste toner discharging operation, the transfer belt may be charged for a predetermined length of time before the toner is supplied to the illuminated surface of the image bearing body.
The controller controls the charging member such that |Vr| greater than |Vc| when the transfer belt is charged for the predetermined length of time. The Vr is a voltage that applied to the charging member when the waste toner discharging operation is performed and Vc is a voltage applied to the charging member when a normal printing operation is performed.
The image forming section may be one of a plurality of image forming sections aligned along a path of the recording medium. The predetermined length of time is a time duration from when a point on the transfer belt is charged by an image forming section most downstream with respect to the path until it is charged by an image forming section most upstream.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.